The Influence of Lubricant Formulation on Surface Damage Under Electrified Rolling-Sliding Contact with Relevance to Electric Vehicle Drivetrain Applications

Surface damage due to electric potential is one of the primary reliability concerns for electric vehicle drive units. This paper for the first time identifies specific lubricant components which promote such damage. The work can help design new e-Fluids that improve EV reliability. Recent experimental studies have shown substantial effects of electric potential across a lubricated contact on contact friction and surface damage; these studies primarily used either pure base oil or fully formulated commercial lubricants. However, the specific behavior of key additive components, such as friction modifiers (FMs), under electric fields remains poorly understood. In this study, a series of model fluids consisting of base oil and single FMs of different types was systematically designed to isolate the effects of individual additives. Friction and wear properties under DC electric field (2 V and <50 mA) and mixed lubrication conditions were comparatively evaluated using a ball on disc tribometer, suitably modified to apply electric potential across the contact. While base oil alone and base oil +MoDTC solution exhibited only mild surface damage, all six solutions tested containing organic friction modifiers (OFMs) showed pronounced groove wear on the cathodic side. Among these, OFMs with amino group (–NH₂), such as oleylamine (OAm), led to the highest friction and wear increase under electrified conditions. A fully formulated e-Fluid containing OAm as a FM exhibited a similar surface damage pattern, despite the presence of other additives in the formulation. Interestingly, this characteristic response was substantially mitigated when the amino group (–NH₂) was replaced with a dimethyl-amino group, –N(CH₃)₂, suggesting that the chemical reactivity and/or steric hindrance of the OFM polar head play a crucial role in the observed phenomena. Based on experimental findings, the underlying wear mechanism is postulated to be electrochemical polishing, a type of corrosive-abrasive wear. It is speculated that OFMs electrochemically react on cathodic metal(oxide) surfaces in the presence of oxygen, promoted by an applied electric field, to form a thin and soft layer that is easily abraded by the oxidized anode surface. This study provides valuable insights into designing electrically robust e-Fluids with desirable tribological properties to improve reliability and efficiency of modern EV drivetrains.

Graphical Abstract